Improve performance of indent-region.

[taku0]

Add jsonian-indent-region and rewrite jsonian-indent-line to speed up indent-region.

When indenting whole large-json-file.json, jsonian-indent-region takes only 6 seconds while indent-region-line-by-line, the default backend for indent-region, takes 5 minutes.

Background:
I'm author of JSON Par, minor mode for structural editing of JSON, which is based on json-mode. As you know, json-mode is abandoned and I couldn't contact the author, I would like to migrate from json-mode to your jsonian.el. JSON Par overrides indent-region-function and other basic functions to improve performance but it should be in the base mode.

[iwahbe]

Hey @taku0. Thanks for the PR. 5 minutes is untenable and 6 seconds seems pretty good. This is definitely an area for improvement for jsonian.

I'm going to keep playing with your code, and I'll do a more through review by the end of the weekend.

From playing with your fork, I see, in addition to the vast speed improvement, an inconsistency.

Given the following (modified) end segment of `large-json-file.json:

                "type": "object",
                "required": [
                    "errors",
                    "id",
                    "location",
                    "msiArmId",
$                     "name",^
                    "provisioningState",
                    "storageAccountArmId",
                    "systemData",
                    "type"
                ]
            }
        }
    }
}

Running jsonian-indent-line with the cursor at $ correctly moves "name", back into line. However, running indent-region with the point at $ and the mark at ^ indents "provisioningState", to match "name",:

                "type": "object",
                "required": [
                    "errors",
                    "id",
                    "location",
                    "msiArmId",
$                     "name",^
                      "provisioningState",
                    "storageAccountArmId",
                    "systemData",
                    "type"
                ]
            }
        }
    }
}

This is obviously not correct. Can you take a look?

[iwahbe]

Would you be willing to explain why your implementation of jsonian--infer-indentation is faster and the intuition behind how it works?

[taku0]

I changed jsonian--infer-indentation not for performance but for robustness. For example, existing implementation doesn't behave well for the following examples (where ■ is the point):

{
  "a":
    1
  ■
}

[
  [
    [

■1
    ]
  ]
]

The new implementation travels from the innermost parent to the outermost ancestor, inferring the indentation from their first child with conditions:

• The open bracket must be at the end of the line (excluding spaces).
• The array/object must not be empty.
• The first child must be before the point.
• The inferred indentation must be greater than zero.

If we cannot infer an indentation, inferring from the outermost ancestor without the third condition.

[iwahbe]

We should be consistent with how we declare variables. parent-position is declared (defaulted to nil), we should do the same with indent and done. This is constant with the rest of the code base.

Suggested change

	(let ((indent nil)
	(origin (point))
	(done nil)
	(let (indent
	(origin (point))
	done

[taku0]

My intention is that if a variable isn't initialized at declaration but needs to be initialized later, I declare it as a bare identifier. Otherwise, i.e. if a variable is initialized to nil-as-false or nil-as-a-not-found-marker, I explicitly initialize it to nil. This is just a quirk of mine growing up as a Java programmer. I will follow whatever styles you prefer.

[taku0]

This is obviously not correct. Can you take a look?

It's an off-by-one error. Fixed.

[taku0]

Maybe I have to adapt your terminology. The following is my understanding. Is it correct?

Term I use	Description	Term you use
Level/nesting level	Number of ancestors of the item.	?
Indentation	How many spaces should we put for the beginning of a line.	Indentation level/indent level
Offset/basic offset	Indentation per level.	Indentation/indent
Member	Array element or key-value pair of object.	Item/element
Key	String before colon in object.	Tag/key
Value	Array element or something after colon in object.	Value

Example:

[
  [
    [
      "abc"
    ]
  ]
]

At string "abc":

• Level: 3
• Offset: 2
• Indentation: 6

[iwahbe]

Sorry for the long delay. I dropped the ball on review.

I have read through the code a couple more times, to the point where I have an at least cursory understanding. As far as I can tell, this is currently an improvement to both accuracy and speed, so I'm going to merge as is.

I'll find some time later to do a cleanup pass to unify naming and comments where my understanding is murky.

Thanks for the PR!